A question of Polarity - Part 2

Last month I dealt with the basics of polarity. This month I hope to show you how to safely deal with the problems which occur in practice.

When in Rome..

In the U.K. we use mainly single pole switching, where only the Live wire is disconnected when equipment is switched off. In the rest of Europe it is normal to use double pole switching. This goes back to a time when many of the supplies in Europe were still D.C. and not referenced to Earth. The advantage of double pole switching is that both Live and Neutral wires are disconnected, which largely makes A.C. polarity irrelevant.

It was this indifference to polarity which led to the development of the familiar reversible 'continental' plug.

When we take a British motorcaravan abroad, we end up mixing the conventions by attaching our 13 amp sockets with their single pole switching to the local mains. Unfortunately most sites in mainland Europe use reversible mains plugs on their hook-ups thus making incorrect connection highly probable.

There are however ways to protect yourself against reversed mains connections.

Most importantly, all installations should have an R.C.D. fitted which monitors the current leaking to earth and should turn off the power in an emergency.

What's an R.C.D?

R.C.D's, which used to be known as Earth Leakage Trips, are another major source of confusion. R.C.D. stands for Residual Current Device. This may be an accurate description, but will be meaningless to most people. What an R.C.D. actually does, is to monitor very accurately the current flowing through the Live and Neutral wires. Because current must flow in a loop, the currents in the two wires should be exactly the same, apart from flowing in opposite directions. Put another way, if the two opposing currents are added together, the total current should be zero.

If the currents are not exactly equal and opposite, some current must be flowing outside of the intended circuit, and a fault must exist. An R.C.D. rated at 30 milliamps will not trip until the current in the Live and Neutral lines is out of balance by more than 30 mA, even though the current on a single conductor may be several amps. This common value of 30 milliamps is chosen because it is small enough not to be fatal in an accident, but sufficiently large to prevent false triggering by the small leakages which do occur in any real installation.

It is vital that the R.C.D. is itself double-pole switching as there is otherwise no protection from reversed mains - some of the cheaper plug-in devices only switch the Live line.

An R.C.D. is however an emergency device and only operates 'after the event'. Far better is to ensure that a problem does not occur in the first place. To be certain, we need some way of finding out whether or not the polarity of our supply is correct.

Which way round?

To be sure of correct polarity we need to test the voltage between each supply line and Earth. An easy way to do this is to use a neon bulb. These little bulbs are often used as mains indicators and consist of two electrodes inside a glass capsule containing neon gas at a very low pressure. When a voltage is applied to the electrodes, an electrical discharge takes place between them which causes the gas to glow in a yellow/amber colour. The type of bulb used for indicators normally will not Óstrike' until about 180 volts is applied across the terminals. This means that if we connect a neon between each supply line and earth, only the one between true Live and Earth will glow. Because the neon glow is Yellow, their light can be filtered to show different colours, and for our purposes we will use one Red neon and one Green one. By connecting them appropriately we get a simple go/no-go display as follows:- 
        Green neon        - Correct polarity
        Red neon          - Reversed polarity
        No neons alight   - No power
        Both neons alight - Severe fault
The latter condition should never occur, but I have known situations where a supply has accidentally been connected between two live phases, giving a voltage of 400v. This can cause a great deal of damage, as well as being a highly lethal.

Logically, you might think that if neither neon lights, it could indicate that there is no Earth connection. In practice however, there will be sufficient capacitive leakage from the vehicle chassis to earth for the indicator to operate, even without a solid Earth.

The actual indicator

The basic indicator is very simple, the Green neon is connected between the Live wire and Earth, whilst the Red neon is connected between the Neutral and Earth. Suitable indicators may be purchased from Maplin, type numbers MK78K (Red) and MK79L (Green) being particularly suitable. These specific indicators are designed to clip into a metal panel which makes them easy to fit. They also have the advantage that the connections are made via insulated 'spade' connectors (Maplin JH81C or JH82D) which means that you do not have to resort to a soldering iron. I should point out that these indicators also contain a resistor to limit the current through the neon. A neon bulb should never be used without a series resistor as the current it can take will burn it out.
 
When I fitted a similar indicator in my Autohomes Kamper, these particular indicators were too long, so I had to use a smaller version (BK52G/BK53H) which did require soldering.

The neon indicators need to be wired 'upstream' of the R.C.D. There are two reasons for this. First, you need to know that the polarity is correct before switching on the main breaker. Secondly, the neons are connected to Earth, therefore the lit neon will itself generate a small amount of earth leakage which may reduce the R.C.D.s sensitivity.
 
To fit the indicator in my Kamper, I first removed the metal panel covering the power distribution. The mains breakers are contained in a plastic box behind the panel, so I had to carefully measure it to find the right locations for the neons. Having marked the panel, I put it back into position and carefully drilled two pilot holes through the panel and into the mains box. With the panel removed again I was able to drill two 7 mm holes to take the neon indicators. The mains box I drilled out to 10 mm in order to receive the body of each indicator.
 
Having connected the wires to the neons, they were passed through the holes and the neons fitted to the panel.

The wires were then passed through the holes on the mains box ready to be connected. One wire from each neon was connected to the appropriate terminal connecting the incoming power to the R.C.D. The neons are not polarity sensitive so it does not matter which of the two wires are used. The remaining wire from each neon was taken to the earth bar to the left of the box where the two wires were twisted together and screwed into the same terminal as an existing cable.
The whole system was then put back together and tested.
 
You may notice that I haven't gone as far as using a switch to correct the polarity if wrong. This has been suggested by several correspondents to Interchange, but I feel that it is not a good idea unless there is some arrangement to prevent the switch being operated whilst 'hot'.

As an additional precaution, a 5000 ohm resistor can be connected between Neutral and Earth, downstream of the R.C.D. This will generate a leakage current of 50 mA if the Live and Neutral are reversed, preventing the R.C.D. from being switched on.